1. Technical Field
The invention relates to substrate fabrication. More particularly, the invention relates to an apparatus and method for providing a uniform distribution of reactive species over a substrate.
2. Description of the Prior Art
During the fabrication of integrated circuits on a silicon wafer, the wafer is coated with a photoresist mask, portions of which are exposed to light and developed to form a pattern on the wafer surface. A plasma containing a reactive gas is then applied to etch the coated wafer. The steps of coating, patterning, and etching may be repeated several times during the fabrication process.
FIG. 1 is a diagram showing the concentration of reactive species during substrate etching relative to the surface of a semiconductor wafer. Reactive species generated from a process gas are applied to the surface of a silicon wafer to expose portions of the wafer surface not coated with a photoresist to the reactive species, such that a desired pattern is etched into the wafer surface.
The etching rate increases with the concentration of reactive species. Due to consumption of reactive species at the wafer surface, the flux of this material has a non-uniform distribution over the etching surface, as shown by the lines identified by the numeric designators 110 and 120. Thus, a region of uniformity 140 occurs toward the center of the wafer 220, while a region of non-uniformity 180 containing excess reactive species occurs toward the edge 160 of the wafer. The region of non-uniformity typically has an increased etch rate.
It is important to maintain a uniform etch rate over the surface of the wafer to ensure the accurate patterning of the wafer surface. Reduction of reactive species at the wafer's edge is one way to achieve this uniformity. The prior art method for reducing this excess reactive species radial flux is to use a focus ring 225 surrounding the wafer, as is shown in FIG. 2. The focus ring forms a diffusive barrier that suppresses the flux of excess reactive species at the wafer's edge.
There are, however, several significant problems arising from the use of a focus ring. The focus ring can introduce contamination during the etching process that may degrade the wafer etching yields, and the subsequent performance of the integrated circuits thus produced. For example, the focus ring itself can react with the reactive species and provide a surface for deposition of by-product. Further, delamination of the films could occur due to poor thermal control and plasma exposure.
Another problem encountered with use of a focus ring is that it makes the insertion and removal of wafers more complex. This slows down the manufacturing process, adding to the cost of production. Further, the likelihood of mishandling and damaging wafers is also increased. It has also been noted that the use of a focus ring distorts the plasma sheath because the electrical field must be perpendicular to the surfaces of the focus ring.
It is also known to use an absorbing annulus to reduce excess reactive species flux. An absorbing annulus is a virtual wafer placed around the periphery of a substrate that is undergoing etching. The absorbing annulus simulates a wafer, thereby consuming excess reactive species, and thereby providing a more uniform etch rate. Unfortunately, the materials used thus far do not produce sufficient effect to improve rate uniformity. Furthermore, the use of certain reactive species, such as silicon, tends to increase the possibility of introducing contamination. Because the annulus is consumable it must be replaced periodically.
It would therefore be a significant advance in the art to provide an apparatus for reducing excess reactive species at the edge of a substrate during etching. It would be a further advance if such an apparatus did not introduce contamination into the fabrication process, and did not increase the difficulty of substrate handling.